This application claims priority to an application entitled xe2x80x9cChannel Spreading Device and Method for CDMA Communication Systemxe2x80x9d filed in the Korean Industrial Property Office on Nov. 17, 1998 and assigned Serial No. 98-49863, the contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates generally to a spreading device and method for a CDMA communication system, and in particular, to a device and method for spreading channels by complex spreading.
2. Description of the Related Art
In general, CDMA (Code Division Multiple Access) communication systems use orthogonal codes for channel separation in order to increase channel capacity. Such a channel separation method using orthogonal codes is typically applied to an IS-95/IS-95A forward link, and can also be applied to a reverse link through time alignment.
However, since future IMT-2000 CDMA communication systems and existing IS-95 CDMA communication systems use different modulation and demodulation methods when orthogonally spreading and despreading signals, there is a compatibility problem between the two systems. The new IMT-2000 systems are capable of several different transmission rates: 1xc3x97 (which bandwidth corresponds to the present IS-95 system), 3xc3x97 (which is three times the bandwidth), 6xc3x97 (six times the bandwidth), 9xc3x97 (nine times the bandwidth), and 12xc3x97 (twelve times the bandwidth). IMT-2000 systems of 3xc3x97 or higher use QPSK (Quadrature Phase Shift Keying) modulation and demodulation for generating orthogonally spread and despread signals, whereas the IS-95 system (and the 1xc3x97 IMT-2000 system) uses BPSK (Binary Phase Shift Keying) modulation and demodulation for generating the orthogonally spread and despread signals. Herein, these orthogonal codes are assumed to be Walsh codes.
This disparity in modulation results in the base stations and mobile stations of one system not being able to communicate with the base stations and mobile stations of the other. This disparity will be described with reference to numerical formulas. When a base station transmitter has orthogonally spread input signals dI and dQ by QPSK modulation using an orthogonal code, i.e., a Walsh orthogonal code Wk, before transmission, a receiver using QPSK demodulation despreads received signals XI and XQ as expressed in Equation (1). When a system using BPSK orthogonal modulation has spread the input signals dI and dQ using the Walsh orthogonal code Wk, a receiver using BPSK demodulation despreads the received signals XI and XQ as expressed in Equation (2).                                           1            2                    ⁢                      (                                          X                I                            +                              jX                Q                                      )                    ⁢                      (                                          W                k                            -                              jW                k                                      )                          =                                            1              2                        ⁢                          (                                                d                  I                                +                                  jd                  Q                                            )                        ⁢                          (                                                W                  k                                +                                  jW                  k                                            )                        ⁢                          (                                                W                  k                                -                                  jW                  k                                            )                                =                      (                                          d                I                            +                              jd                Q                                      )                                              (        1        )            xe2x80x83(XI+jXQ)Wk=(dI+jdQ)WkWk=(dI+jdQ)xe2x80x83xe2x80x83(2)
Therefore, because the two systems use different modulation and demodulation methods for generating orthogonal spreading and despreading signals, the two systems are incompatible, disabling communication between them. That is, IS-95 mobile stations (and 1xc3x97 mobile stations of the IMT-2000 system) cannot communicate with an over-3xc3x97 base station of the IMT-2000 system, and the over-3xc3x97 IMT-2000 mobile stations cannot communicate with an IS-95 base station. To be exact, when a base-station transmits a signal spread by QPSK modulation and then a mobile station despreads the channel spread signal by BPSK demodulation, the relationship between the input and output of the mobile station demodulator can be expressed as:                                           1            2                    ⁢                      (                                          X                I                            +                              jX                Q                                      )                    ⁢                      W            k                          =                                            1              2                        ⁢                          (                                                d                  I                                +                                  jd                  Q                                            )                        ⁢                          (                                                W                  k                                +                                  jW                  k                                            )                        ⁢                          W              k                                =                                    (                                                d                  I                                -                                  jd                  Q                                            )                        +                          j              ⁡                              (                                                      d                    I                                    +                                      jd                    Q                                                  )                                                                        (        3        )            
Equation (3) demonstrates that when the base station orthogonally spreads a transmission signal by QPSK modulation before transmission and the mobile station despreads the spread signal by BPSK demodulation, the signal demodulated by the mobile station by QPSK demodulation becomes not dI+jdQ, but (dIxe2x88x92jdQ)+j(dI+jdQ). Therefore, when the QPSK modulated signal undergoes BPSK demodulation, communication cannot be performed between the base station and the mobile station. Alternatively, communication cannot also be performed between a base station which spreads a channel by BPSK modulation and a mobile station which despreads the spread channel by QPSK demodulation.
However, it is necessary to maintain backwards compatibility so that the existing IS-95 mobile stations can be provided with communication services even when the future IMT-2000 CDMA communication system is being implemented, so that the mobile stations of the IMT-2000 system can communicate with the base stations of the IS-95 system.
FIG. 1 shows the IS-95/IS-95A forward link in which channels are separated by Walsh orthogonal codes. Referring to FIG. 1, channels are separated by unique Walsh orthogonal codes Wi (where i=0 to 63), respectively. The IS-95/IS-95A forward link uses rate R=1/2 convolutional codes for channel coding, employs BPSK modulation for spreading the Walsh orthogonal codes, and has a bandwidth of 1.2288 MHz. Accordingly, the number of available channels is 1.2288 MHz/(9.6 KHz*2)=64. That is, the IS-95/IS-95A forward link can separate 64 channels using the Walsh orthogonal codes.
Therefore, the number of available Walsh orthogonal codes is dependent on the employed modulation method arid the minimum data rate. However, future CDMA mobile communication systems will require a greater number of channels assigned to users in order to improve performance. To this end, future CDMA mobile communication systems will employ traffic channels, pilot channels and control channels, thereby increasing channel capacity.
However, there are a limited number of available orthogonal codes available for use. This limitation will restrict the increase in channel capacity. To overcome this disadvantage, it is desirable to generate quasi-orthogonal codes, which will limit interference with the orthogonal codes and have a variable data rite. The quasi-orthogonal code is disclosed in detail in Korean patent application No. 97-47457, filed by the applicant, and a complex quasi-orthogonal code is disclosed in Korean patent application No. 98-37453, also filed by the applicant.
In order to perform orthogonal spreading and despreading using the complex quasi-orthogonal sequence, the IMT-2000 CDMA communication system using the quasi-orthogonal code of the complex quasi-orthogonal sequence employs QPSK orthogonal modulation. Thus, when the Walsh orthogonal codes undergo QPSK modulation, the spreading scheme for specific common channels such as pilot channels and sync channels cannot maintain backward compatibility with the existing IS-95 system employing BPSK modulation.
It is, therefore, an object of the present invention to provide a spreading device and method for enabling communication between a base station and a mobile station having different channel spreading and despreading schemes in a CDMA communication system.
It is another object of the present invention to provide a device and method for selectively performing orthogonal spreading by QPSK modulation or BPSK modulation in a CDMA communication system.
It is further another object of the present invention to provide a device and method for enabling a base station to perform orthogonal spreading on a specific channel by BPSK modulation and perform orthogonal spreading on other channels by QPSK modulation in a CDMA communication system.
It is yet another object of the present invention to provide a device and method for enabling a mobile station to perform orthogonal despreading on a specific channel by BPSK demodulation and perform orthogonal despreading on other channels by QPSK demodulation in a CDMA communication system.
It is yet another object of the present invention to provide a device and method for enabling a base station to perform orthogonal spreading on a specific channel by BPSK modulation and perform orthogonal spreading on other channels by QPSK modulation, and enabling a mobile station to perform orthogonal despreading on a specific channel by BPSK demodulation and perform orthogonal despreading on other channels by QPSK demodulation in a CDMA communication system.
It is still another object of the present invention to provide an orthogonal spreading device and method wherein channel transmitters include a BPSK orthogonal spreader and a QPSK orthogonal spreader to selectively spread transmission signals by BPSK modulation or QPSK modulation in a CDMA communication system.
It is still another object of the present invention to provide an orthogonal despreading device and method wherein channel receivers include a BPSK orthogonal despreader and a QPSK orthogonal despreader to selectively despread received signals by BPSK demodulation or QPSK demodulation in a CDMA communication system.
It is still another object of the present invention to provide a device and method wherein channel transmitters include an orthogonal spreader for orthogonally spreading transmission signals by BPSK modulation using a Walsh orthogonal code and an orthogonal spreader for orthogonally spreading transmission signals by QPSK modulation using a Walsh orthogonal code or a quasi-orthogonal code, thereby to selectively spread the transmission signals by BPSK modulation or QPSK modulation in a CDMA communication system.
It is still another object of the present invention to provide a device and method wherein channel receivers include an orthogonal despreader for orthogonally despreading spread signals by BPSK demodulation using a Walsh orthogonal code and an orthogonal despreader for orthogonally despreading spread signals by QPSK demodulation using a Walsh orthogonal code or a quasi-orthogonal code, thereby to selectively despread the spread signals by BPSK demodulation or QPSK demodulation in a CDMA communication system.
To achieve the above and other objects, there is provided a channel spreading device for a base station in a CDMA communication system. The device comprises a spreading code generator for generating a real-component spreading code and an imaginary-component spreading code corresponding to a designated channel spreading code index; a circuit receiving a mode control signal, for disabling generation of the imaginary-component spreading code when the received mode control signal indicates the BPSK (Binary Phase Shift Keying) modulation mode; and a complex multiplier receiving a channel signal, for complex multiplying the received channel signal by the spreading codes to spread the channel signal.
The spreading code generator comprises a controller for generating a quasi-orthogonal code mask index and a Walsh orthogonal code index corresponding to the designated channel spreading code index; a real-component spreading code generator for generating a real-component quasi-orthogonal code mask corresponding to the quasi-orthogonal code mask index, generating a real-component Walsh orthogonal code corresponding to the Walsh orthogonal code index, and generating a real-component channel spreading code by multiplying the real-component quasi-orthogonal code mask by the real-component Walsh orthogonal code; an imaginary-component spreading code generator for generating an imaginary-component quasi-orthogonal code mask corresponding to the quasi-orthogonal code mask index, generating an imaginary-component Walsh orthogonal code corresponding to the Walsh orthogonal code index, and generating an imaginary-component channel spreading code by multiplying the imaginary-component quasi-orthogonal code mask by the imaginary-component Walsh orthogonal code.